Recently, a device having a two-dimensional electron gas channel at a hetero interface has received substantial attention because of its excellent performances in high-frequency and high-power drive. The GaN transistors are manufactured by processing a epitaxially grown semiconductor crystal by lithography technique. At that time, a gate insulator or/and a passivation film can be stacked on top of the crystal according to a purpose.
The gate insulator is a protective film inserted between a gate metal and a semiconductor crystal for the purpose of preventing a leak current on the gate. In general, it is known that a Schottky electrode formed on a nitride semiconductor exhibits a larger leak current than theoretically expected, and the gate insulator is often introduced to improve the leak characteristics.
The passivation film is a protective film stacked on top of the crystal for the purpose of stabilizing electrical properties of the crystal surface. Unstable electrical property on the crystal surface can lead to a reduction in output and fluctuations in threshold voltage thorough transient responses such as a gate lag, a drain lag, or current collapse phenomena. Accordingly, the film introduced to prevent such degradations as needed.
For the purpose of simplifying manufacturing steps and reducing manufacturing cost, it is often the case that insulator and a dielectric film are commonly used for both protection. In a conventional method, the dielectric film has been over stacked by a thermal CVD, plasma CVD, cat-CVD (catalytic chemical vapor deposition) or the like on the a nitride semiconductor crystal layer that is grown by crystal growth techniques such as metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or the like. Where, an acid treatment is carried out on a nitride crystal surface to remove a natural oxide formed under the atmosphere exposure during the substrate transfer from the crystal growth chamber to the CVD chamber. For example, in P. Kordos et al., Applied Physics Letters 87, 143501 (2005), there is disclosed a GaN-MISHFET in which a SiO dielectric material is provided on a semiconductor surface by plasma CVD.
This example shows the result that a gate leak current was reduced by introducing the dielectric film. However, in the conventional method including this method, it is impossible to completely remove the natural oxide at an interface between the semiconductor crystal and the insulator. Additionally, it suffers nitrogen vacancies formation on a nitride crystal surface. The nitrogen vacancy can be the factor of electrically unstable surface by forming intermediate level. Hence, the transistor manufactured by the conventional method often exhibits a drain lag, a gate lag, or the like. That is, in accordance with the conventional methods, an effect of passivation by the dielectric film has been insufficient, and therefore there has been a problem in putting the methods into practical use.